Electric fluid heater

ABSTRACT

The invention is directed to an electric resistance heating element for heating flowing media. It is the object of the invention to provide an arrangement for electric resistance heating elements which can be implemented using simple technology and by which high power densities can be achieved at the same time. In an electric fluid heater comprising ceramic material or polymer composite material with a plurality of channels through which the medium to be heated flows, the above-stated object is met according to the invention in that the fluid heater comprises at least one heating element which is provided with at least one channel, in that the heating element has electrode surfaces on opposite outer surfaces so that there is a flow of current between the electrode surfaces substantially transverse to the direction of the channels, and in that at least one of the two electrode surfaces of the heating element has a connection surface which is arranged on the adjoining outer surface of the heating element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of International Application No. PCT/DE 2005/000586, filed Mar. 31, 2005 and German Application No. 10 2004 016 434.7, filed Mar. 31, 2004, the complete disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to an electric resistance heating element for heating flowing media.

b) Description of the Related Art

Electric resistance heating elements in the form of honeycomb bodies, tube bundles or multi-hole plates are often used for heating flowing media such as air or nonconductive liquids such as silicon oil, glycol, hydraulic oil, gasoline, or diesel fuel. Generally, there is a plurality of channels or bore holes in the resistance element through which the medium (fluid) to be heated can flow or is pumped. Heating current flows through the volume of the resistance heating element more or less homogeneously and heats it. A heat exchange takes place between the resistance element and the fluid at the surface of the channels or bore holes. The amount of heat delivered to the flowing medium per time unit depends, among other things, on the temperature difference between the resistance body and the fluid, the size of the heat exchange surface, the thermal capacity of the medium, and its flow rate. Of course, the resistance element can only deliver as much heat per time unit to the medium to be heated as it can convert to electric power P at the available operating voltage U. The power P is given by P=U²/R. This means that for high heating outputs the resistance heating element must have a low electric resistance R. The resistance of the heating element is determined by the resistivity ρ of the material, the cross-sectional area A of the heater, and the length l of the circuit path or current path: R=ρ*l/A.

In the prior art, for reasons relating to simple technical handling, prismatic honeycomb bodies or tube bundles are metallized, e.g., by screen-printing or rolling, for electric bonding with their oppositely located sides. As a result, the electrode surface A is equal to the end face of the honeycomb body less the sum of the cross-sectional areas of the channels through which the medium flows. The length l of the current path is identical to the length of the channels. The resistivity of the resistance materials cannot be reduced at will; in particular for ceramic resistance bodies with PTC characteristics, the lower limit reached in practice is about 5 to 10Ω*cm. This leads to two conflicts in the production of honeycombs, tube bundles or multichannel heaters. For a good heat transfer, a sufficiently large channel length is aimed for, but this increases the electric resistance and limits the heating output. For a low flow resistance of the honeycomb heater, the proportion of surface area of the honeycomb through which flow takes place should be as high as possible. However, this means that the proportion of the surface area that is available for the connection electrodes is smaller, which additionally limits the electric power consumption.

To solve this conflict, it is proposed in DE 100 60.301 A1, for example, to metallize the channels of the honeycomb body internally, wherein adjacent channels have different polarities and the length of the current path is equal to the wall thickness of the channels. These suggested solutions require elaborate technical measures to ensure sufficiently large insulating distances or creepage distances between electrodes of different polarity. Further, small wall thicknesses which are actually advantageous with respect to flow can lead to failures due to voltage breakdown. In addition, internal metallization in honeycomb bodies leads to the problem that the cell width has a lower limit and the channel length has an upper limit for technical reasons.

DE 102 01 262 A1 describes a honeycomb body which is cut into two halves of equal size parallel to the electrode surfaces. The cut surfaces are metallized, connected to a pole of the voltage source and then joined again. Simultaneously, the second pole of the voltage source is applied to the two outer electrode surfaces of this sandwich arrangement. The honeycomb halves which are connected in parallel in this way have four-times less electrical resistance than the simple honeycomb due to the doubling of the electrode surface while simultaneously halving the conductor length. This arrangement is disadvantageous owing to the high production cost for the additional cutting and metallizing steps and due to the fact that there are now joints inside the channels. Substantial disturbances in the flow can occur at these joints due to geometric tolerances which are difficult to avoid.

OBJECT AND SUMMARY OF THE INVENTION

It is the primary object of the invention to provide an arrangement for electric resistance heating elements which can be implemented using simple technology and by which high power densities can be achieved at the same time. It should be possible to tailor the channel cross section and channel length to the respective application in technical respects relating to heat and flow without being limited by metallization technique.

In an electric fluid heater comprising ceramic material or polymer composite material with a plurality of channels through which the medium to be heated flows, the above-stated object is met according to the invention in that the fluid heater comprises at least one heating element which is provided with at least one channel, in that the heating element has electrode surfaces on opposite outer surfaces so that there is a flow of current between the electrode surfaces substantially transverse to the direction of the channels, and in that at least one of the two electrode surfaces of the heating element has a connection surface which is arranged on the adjoining outer surface of the heating element. In arranging the connection surfaces, a more or less broad insulating margin suited to the respective application must be maintained relative to the other respective electrode surface. The special advantage of the arrangement according to the invention consists in that any quantity of identically produced heating elements, each of which is rotated by 180° around the surface normal of the electrode surfaces, can be arranged successively. By rotating and successively joining the heating elements, these heating elements can be assembled in the form of a parallel circuit by the connection surfaces to produce an electric fluid heater. The connection can be carried out by clamping, gluing or soldering. In so doing, the heating elements are oriented exactly parallel.

In an advantageous construction according to the invention, the electric connection leads can serve at the same time to mechanically fixate the heating elements. The flow of current through the honeycomb body takes place transverse to the flow direction of the fluid. The electrode spacing is given by the sum of the cell width plus twice the wall thickness. Therefore, it is shorter by a multiple compared to front-side electric bonding. However, it does not reach the critical values that are reached with internal metallization.

The process is more dependable compared to internal metallization because any holes or weak points that may exist in individual honeycomb walls cannot lead directly to short circuiting or voltage breakdown. The parallel connection of the comparatively short current paths through the channel walls of individual segments and the additional parallel connection of the segments lead to low resistance values. By means of the arrangement according to the invention, it is possible to produce profiles in the form of single-row honeycomb segments by extrusion. The latter can be cut to length precisely and inexpensively after sintering. Proven methods of screen-printing or sputtering can be used for the metallization for applying the electrode surfaces.

The invention will be described more fully in the following with reference to embodiment examples.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic illustration of a heating element; and

FIG. 2 shows an exploded view of the electric fluid heater according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As can be seen from FIG. 1, the essential components of the heating element I are the channels 2 and the two oppositely located electrode surfaces 3, 4. The first electrode surface 3 communicates with the connection surface 5. The second electrode surface 4 does not communicate with any connection surface 5 in this embodiment form. However, it is easily possible to provide this second electrode surface 4 with a second connection surface which, however, would be located exactly opposite from the first connection surface 5. The molded body of the heating element 1 is preferably made of PTC ceramic based on semiconducting barium titanate and prepared through the addition of organic binding agents and plasticizers to form a stiff-plastic mass, from which the prismatic molded bodies are produced with a series of parallel channels 2 by extrusion and then sintered. After sintering, they are cut to length by diamond cutting wheels. By means of screen-printing, metal spraying, or sputtering, the molded bodies are provided with suitably structured metal electrodes, for example, of aluminum or silver.

The heating elements 1 produced in this way are rotated by 180° with reference to the surface normal of the electrode surfaces 3, 4 and arranged successively. This results in a stack of heating elements 1, the first electrode surface 3 being electrically bonded in each instance with the second electrode surface 4 of the adjacent heating element 1.

As can be seen from FIG. 2, the electric bonding to the first electrode surfaces 3 and therefore also the electric bonding to the contacting second electrode surfaces 4 via the connection surfaces 5 is produced by means of the top lead 6, e.g., by soldering. Similarly, electrical bonding to the first electrode surfaces 3 and accordingly again to the second electrode surfaces 4 via the connection surfaces 5 is produced by the bottom lead 7. The connection can also be carried out by gluing or clamping in addition to soldering. A parallel connection of the heating elements 1 is realized through precisely this construction. In order to prevent short circuiting between the electrode surfaces 4 and the connection surfaces 5 and leads 6, 7, appropriate steps for insulation are carried out in a known manner.

For example, when a PTC ceramic material with a cold resistivity of 10 Ω*cm is molded to produce a heating element 1 according to FIG. 1 with six parallel channels 2 having a channel length of 20 mm and a channel cross section of 2×2 mm and a wall thickness of 0.4 mm, a cold resistance of about 5 Ω is achieved when metallizing with lateral connection electrodes according to FIG. 2. The parallel connection of five honeycomb segments of this kind gives a total resistance of 1 Ω. Therefore, at an operating voltage of 12 V, a power consumption of 144 Watts is possible.

While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.

REFERENCE NUMBERS

-   1 heating element -   2 channel -   3 first electrode surface -   4 second electrode surface -   5 connection surface -   6 top lead -   7 bottom lead 

1-7. (canceled)
 8. An electric fluid heater comprising: ceramic material or polymer composite material; said fluid heater further comprising at least one heating element; each heating element being provided with at least one channel through which the medium to be heated flows; each heating element having electrode surfaces on opposite outer surfaces so that there is a flow of current between the electrode surfaces substantially transverse to the longitudinal axis of the channels; and at least one electrode surface of each heating element having a connection surface which is arranged on the adjoining outer surface of the heating element.
 9. The electric fluid heater according to claim 8, wherein the heating elements are constructed as flat prismatic bodies, each having a plurality of parallel channels whose longitudinal axes lie in a plane.
 10. The electric fluid heater according to claim 8, wherein the heating elements are arranged successively, each heating element being rotated by 180° around the surface normal of the electrode surfaces, and the connection surfaces are connected to the positive pole or negative pole of the voltage supply via electric leads by clamping, soldering or gluing.
 11. The electric fluid heater according to claim 8, wherein the heating elements are arranged successively, each heating element being rotated by 180° around the surface normal of the electrode surfaces, and the connection surfaces are connected to the phase terminal and neutral terminal of an AC voltage source via electric leads by clamping, soldering or gluing.
 12. The electric fluid heater according to claim 8, wherein the heating elements comprise a material having a positive temperature coefficient of the electric resistance in a preselectable temperature interval.
 13. The electric fluid heater according to claim 8, wherein the heating element comprises semiconducting barium titanate ceramic with a positive temperature coefficient of the electric resistance.
 14. The electric fluid heater according to claim 8, wherein the heating element comprises a polymer composite material with a positive temperature coefficient of the electric resistance. 